The present invention relates to a light source device for a projector, which is installed in a projector apparatus equipped with a spatial modulation element such as a liquid crystal display device and a digital micro-mirror device.
In recent years, a projector apparatus, which is equipped with a spatial modulation element such as a liquid crystal display device and a digital micro-mirror device, has been developed. A color sequential display type projector apparatus is known as such a projector apparatus, in which lights consisting of the three primary colors are sequentially formed bypassing white light emitted from a white light source through a color wheel which is driven to rotate, and a full color image is displayed by irradiating each pixel with each of the three primary color lights one by one (Refer to Japanese Patent No. 3896074).
FIG. 7 shows the structure of an example of such a color sequential display type projector apparatus according to the prior art. The projector apparatus 80 comprises a light source lamp 81 which emits white light, an elliptical reflection mirror 82 which condenses the light from light source lamp 81, a color wheel 83, which is driven to rotate, and which divides, in terms of time, the light from the light source lamp 81, thereby sequentially forming lights consisting of the three primary color and white light, a rod integrator 84 for receiving and making uniform the light which passes through the color wheel 83, a flat reflection mirror 85 which reflects the light emitted from the rod integrator 84, a reflection type spatial modulation element 86 which the light from the flat reflection mirror 85 enters so that a light image is emitted therefrom, a spatial modulation element actuator 87 which drives the spatial modulation element 86, and a projection lens 88 which projects the light image emitted from the spatial modulation element 86 on a screen. The color wheel 83 comprises a filter element 83b which transmits blue light, a filter element 83r which transmits red light, a filter element 83g which transmits green light, and a filter element 83w which transmits white light, wherein these filter elements are respectively in a shape of a partial sector, and are aligned so as to form an annular shape on the same plane.
In such a projector apparatus 80, when the white light from the light source lamp 81 passes through the color wheel 83 which is driven to rotate, the white light is filtered by the filter element that is in the optical path of the white light, the filter element changing sequentially according to the rotation of the color wheel 83, thereby sequentially forming for a period of time white light and three primary color lights which are blue light, red light and green light. Then, when these primary color lights and the white light enter the spatial modulation element 86 one by one through the rod integrator 84 and the flat reflection mirror 85, a light image, which is made up of the respective primary color lights and the white light, is formed. And when the light image made up of the respective primary color lights and the white light which are emitted from the spatial modulation element 86 is projected sequentially on the screen through the projection lens 88, a desired color image is formed on the screen.
However, the projector apparatus 80 described above has the following problems:
(1) Because the light from the light source lamp 81 is filtered in each period of time into respective primary color lights and white light by the color wheel 83, the light intensity which is actually projected is, for example, a half (½) of the light intensity of light emitted from the light source lamp 81, so that the usage efficiency of the light is very low. As shown in FIG. 8, the projection time for forming one frame of an image to be displayed, is divided into four time divisions S1-S4 which sequentially continue in terms of time. When the color wheel 83 is operated, for example, blue light enters the space modulation element 86 in the time division S1, red light enters it in the time division S2, green light enters it in the time division S3, and white light enters it in the time division S4. In the time division S4, approximately all color light components emitted from the light source lamp 81 pass through the color wheel 83 and are used (the intensity of light used therein is one quarter (¼) of that in the entire time divisions S1-S4). Although, in the time division S1, blue light among the light components emitted from the light source lamp 81 passes through the color wheel 83 and is used, the other color light components are wasted (the intensity of light used therein is 1/12 of that in the entire time divisions S1-S4). Moreover, although, in the time division S2, red light among the light components emitted from the light source lamp 81 passes through the color wheel 83 and is used, the other color light components are wasted (the intensity of light used therein is 1/12 of that in the entire time divisions S1-S4). Furthermore, although, in the time division S3, green light among light components emitted from the light source lamp 81 passes through the color wheel 83 and is used, the other color light components are wasted (the intensity of light used therein is 1/12 of that in the entire time divisions S1-S4). Thus, in the above projector apparatus 80, the usage efficiency of light is extremely low, that is, as low as ½ (¼+ 1/12+ 1/12+ 1/12), and it is difficult to configure the structure of a projector apparatus capable of obtaining high illuminance with low electric power.
(2) Generally, in such a projector apparatus which projects a full color image, the brightness of each primary color light is controlled by changing the reflex time of the light (or the transmission time if a transmission type space modulation element is used) with respect to a space modulation element, wherein the number of gradation levels of each primary color light is 256 (8 bits), so that it becomes possible to reproduce color with gradation levels of the third power of 256 (16,777,216). And in order to realize higher color reproduction quality, an increase of the number of gradation levels of each primary color light is required. However, it is difficult to increase the number of gradation levels because of the response speed of the space modulation element 86 in the above projector apparatus. In more detail, in the projector apparatus, the projection time for forming one frame of an image to be displayed is, for example, 1/60 second (60 frames per second). And in order to realize the 256 gradation levels in each primary color light, it is required that the space modulation element 86 be operated 256 times in each of the time divisions S1-S3 where respective primary color lights are projected within the projection time of a frame, the response time required for the space modulation element 86 is 1/(60×4×256) second (16.2 microseconds). And since the minimum response speed of known space modulation elements is approximately 15 microseconds even when a digital micro-mirror device is used as the space modulation element 86, the number of gradation levels cannot be increased, so that it is difficult to obtain higher color reproduction quality.